1. Field of Endeavor
The present invention relates to geologic reservoir stimulation and more particularly to geologic reservoir stimulation by explosively-augmented fracturing in wellbore sidetracks.
2. State of Technology
U.S. Pat. No. 6,732,799 for apparatus for stimulating oil extraction by increasing oil well permeability using specialized explosive detonating cord provides the state of technology information reproduced below. The disclosure of U.S. Pat. No. 6,732,799 is incorporated herein in its entirety by this reference.
Oil wells have been known to produce oil for nearly seventy-five (75) years. Oil wells that have been producing oil for several years often experience a reduction in oil extraction or production as the years progress. When the oil production is reduced, remedial action in the form of stimulation to improve the oil production output of the oil well is undertaken.
Generally, such stimulation may involve improvement of the permeability or transmissibility of the reservoir itself or merely clearing the casing perforations of accumulated production-restricting contaminants, such as heavy hydrocarbons, paraffins, tars, mineral depositions, or formational fines in or near the casing perforations, by the use of vibratory explosive forces created by the ignition of a detonator and detonating cord.
Typically, the methods used to increase the transmissibility of sand, shale or rock formation are shock treatments using explosives, acid washes, hydraulic fracturing, and high energy gas fracturing.
The flow rate of a fluid such as oil through a porous medium, such as a sand, shale or rock formation, is a function of the permeability or transmissibility of that particular formation. If the transmissibility of oil from an oil bearing formational reservoir can be increased, more fluid can be recovered. It is well known that over the life of an oil or gas well, with continued pumping or removal of the oil or gas from that well, the permeability of the surrounding formation may be economically insufficient to justify continued production, even though a large percentage of fluid hydrocarbons remain. When this occurs, the oil well operator can either abandon the oil well or can attempt to increase the permeability of that formation to rejuvenate the flow of liquid hydrocarbons there through.
There are currently a number of techniques or processes for mechanically increasing permeability. The best known processes are: (1) hydraulic fracturing; (2) explosive fracturing; (3) acidizing, and (4) high energy gas fracturing.
Hydraulic Fracturing
Hydraulic fracturing is a process used for increasing the permeability of a rock formation by a slow introduction of a highly viscous fluid that is pumped into the area of a well bore between packings. In the hydraulic fracturing technique, the combined fluid pressure is steadily increased until the tensile strength of that particular rock material is exceeded. When this occurs, a fracture will be initiated which propagates from opposite sides of the well bore into the formation; this is known as a biwing fracture. This fracture is induced at a point of least resistance in the rock material.
A fluid used in practicing such a method is one selected to be sufficiently viscous to enable the suspension and mass transport of proppants suspended therein. Such proppant materials are either sand grains or grains of a synthetic material and are made to pass into and settle in the induced fracture. So arranged, the proppants prevent the induced fracture from totally closing once the pressure on the fluid is reduced and the normal closing pressures of the rock formation are re-exerted. Hydraulic fracturing generally involves the generation of the single biwing fracture that extends in a vertical plane from opposite sides of the well bore into the rock formation. In such fracturing, the injected fluids will, by and large, remain in the formation, and the proppants used to support the fracture may, due to compaction, actually come to restrict the permeability of that rock formation rather than enhance or improve its permeability. Another drawback to the use of hydraulic fracturing, and of major consideration in selecting a rock formation fracturing process, is the extent and expense of the equipment and labor involved, since the hydraulic fracturing method requires the use of hydraulic pumps with a high pressure capability along with the temporary positioning of a packer above the oil bearing strata.
Explosive Fracturing
In an attempt to overcome the limitations of hydraulic fracturing where generally only a single biwing fracture is produced, explosives have been used for dynamically loading a rock formation. Because of the speed of burning of an explosive, and the shock wave produced thereby, it has been found that explosive compaction of the formation rock around the well bore opposite the explosion may actually decrease rather than increase the permeability of the rock formation. Therefore, while explosive fracturing may provide a greater circumferential fracturing effect in a rock formation, it may also depredate the permeability of the rock formation to the point where most, if not all, permeability is lost. Explosive fracturing has been, therefore in the past, generally considered unpredictable and unreliable.
Acid Fracturing
Acid fracturing is a process which is utilized to increase permeability by dissolving reactive materials in a rock formation to create conductive passageways or “worm holes” and for chemically etching the oppositely disposed faces of a rock formation fracture. The acids which are frequently used are concentrated solutions of hydrofluoric and hydrochloric acid, either of which can, of course, create serious safety problems in the transportation and conveyance of such highly corrosive fluids to a desired location in an oil well bore.
Furthermore, acidizing is limited by a danger of formation matrix collapse due to excessive rock dissolution near the well bore as a consequence of a preferential invasion of the acid used into zones of high, rather than low, permeability.
Another limitation found in the use of the acidizing technique, is that the depth of penetration is limited by the type of rock in the rock formation and the degree of the strength of the acid. Many times, these acidizing processes have been found to cause extensive damage to the well bore due to the geochemical reactions produced. Therefore, the nature of the materials at the location where the fracture is to be induced must be identified prior to selection of the acid to be used. Where such unwanted geochemical reactions take place, they can create damage, leading even to a loss of permeability.
High Energy Gas Fracturing
Propellant deflagration is a recent technology that has been developed to produce a good distribution of fractures in the oil-bearing rock formation around a well bore without the problems that have been inherent in the explosive and acid processes.
In the use of high energy gas fracturing, a significant amount of high energy is created by a deflagrated propellant that is ignited in a well bore adjacent to a rock formation to be fractured. Upon ignition of the propellant in the canister, high-energy gas and other products of this combustion process, such as water vapor or steam, are driven to near sonic velocities.
The propellant can be burned radially from a longitudinal center cavity within the propellant, or can be burned from one end, as in a cigarette burn, or a combination of both processes can be employed to develop the high energy fracturing process.
In practice, high-energy gas fracturing involves the placement of a canister of a propellant adjacent to a perforated wall of a well bore in the zone where it is desired to increase the permeability of the oil-bearing rock formation. An igniter rod is then implanted adjacent to the canister containing the propellant. To ignite the propellant, an electrical current is transmitted over one or more electrical wires from the surface above the entrance to the oil well bore to instantaneously detonate an electric blasting cap which initiates deflagration thereof in a period of milliseconds. Once deflagration occurs, a high volume of pressurized gas and water is generated at near sonic velocities. By such deflagration, the energy loading in the oil well bore will be propagated much faster than that which occurs during hydraulic fracturing. Such an increase in the propagation speed of the energy loading produces multiple fractures in directions other than in the plane of least resistance through the oil-bearing rock formation surrounding the oil well bore. The propellant is selected from a group of propellants which will burn at a far slower rate than those propellants used for typical explosive detonations. No destructive shock wave will, therefore, be generated in a propellant deflagration which would cause crumbling of the material around the well bore.
U.S. Pat. No. 3,771,600 for a method of explosively fracturing from drain holes using reflective fractures provides the state of technology information reproduced below. The disclosure of U.S. Pat. No. 3,771,600 is incorporated herein in its entirety by this reference.
Extraction of oil or gas as well as the leaching of underground minerals is often complicated by the lack of permeability in the formation. In order to maximize production from such low permeability formation, it is often necessary to fracture the formation and thereby increase permeability. There are two basic methods of creating fractures in a formation. One is to create hydraulic fractures by applying a pressurized fluid against the formation until the formation parts. Another method is to detonate explosives in the formation or wellbore to create a shock wave which fractures the rock matrix of the formation.
Explosive well stimulation has been used for many years. However, explosive stimulation has not been entirely successful. As a result, hydraulic fracturing introduced over 20 years ago has been the standard stimulation mode, due mainly to the high degree of success of this method. Recently, however, new interest in stimulating wells with explosives has been generated by the development of improved explosives and new methods of using them. There are presently two basic methods of explosive fracturing. One is to detonate the explosive in the wellbore, and the other is to detonate the explosive in the formation adjacent to the wellbore. A method of detonating explosives in the formation adjacent the wellbore is to hydraulically fracture the formation, and then load the fracture zone with an explosive material.
When the explosive is confined in the wellbore, the result of detonation is a cylindrical rubble zone in the vicinity of the wellbore surrounded by a system of vertical fractures radiating like wheel spokes from the rubble zone. This result is achieved by the explosive undergoing a very rapid self-propagating decomposition. This decomposition yields more stable products in the form of gases which exert tremendous pressure as they expand at the high temperature generated by the release of heat. This rapid release of energy creates a shock wave.
The rock matrix, adjacent to an explosive charge, will be shattered as the shock wave moves through it. The shock wave consists of two components, compression wave and a shear wave. When the energy level of either of these waves exceeds the strength of the rock under dynamic loading, the rock will fail, thus creating a fracture network. The gases generated in the explosion obtain a pressure on the order of one million pounds per square inch, which pushes against the exposed surfaces of the fractured rock matrix. The expansion of gases will extend the fractures until its energy for doing work is dissipated.
When the explosives are placed in the formation, usually in a fracture created by hydraulic means, a rubble zone will be created in the fracture area upon detonation of the explosive. When an explosive is located in a horizontal fracture and detonated, a high pressure shock wave shatters the adjacent surfaces of the fracture. as the shock wave moves upward and downward from the plane of detonation, it will traverse various strata. As the wave moves through a density discontinuity, part of the wave will be reflected back as a tension wave. The tensile strength of rock is several orders of magnitude less than the compressive strength, therefore new fractures will be created by the tension wave.
Explosive detonations occurring in vertical fractures yield similar results as those occurring in horizontal fractures. Lateral expansion of the original fracture occurs more readily, however, since the vertical height of the fracture is confined by the stratographic boundaries, thus requiring a smaller volumetric increase for fracture extension. Placement of explosives in fractures is not entirely satisfactory due to the limited amount of explosives that can be placed in a fracture created by hydraulic means. The width of fracture controls the net thickness of the explosive layer and thus limits the volume of gas products available for fracture extension. Since the explosive is present as a thin layer, a limited quantity of gas is available per unit surface area of the fracture. It thus can be seen that it will be advantageous to be able to place a larger volume of explosive in the formation. Additionally it is preferable to locate the explosive in the formation rather than in the wellbore so as to prevent wellbore damage and sloughing of the formation adjacent the wellbore.
Since the tensile strength of rock is appreciably less than its compressive strength, it would be preferable to devise a process of explosive fracturing which utilizes a tension wave to a greater degree than is now being practiced. The use of more explosives in the formation together with greater use of tension waves would result in more effective stimulation of the formation.
One method of providing space for more explosives in the formation is by use of drain holes. Drain holes are simply boreholes drilled along a horizontal plane into the formation being produced to provide for more efficient recovery through increased drainage area. The history of drain holes goes back past the turn of the century with early work done in the 1930's. This work was largely unsuccessful due to the economics of the reservoir in which it was used. Revival of drain holes occurred in the 1950's for a brief period and had some success.
Since a 5½ inch hole can be easily drilled by presently known drain hole drilling methods, it can readily be seen that s significantly increased amount of explosives can be located in such a drain hole. Since windows can be cut in the casing and drain holes drilled through the casing window, drain hole drilling is not limited to new wells. Since explosive stimulation is often used in fields that have already been drilled, the casing window feature of drain holes is extremely advantageous.